We calculate the relative magnetic helicity [1]}, [2]} in
the rectangular box \(V=\lbrace x,y,z\,\mbox{:}\,-l_x\le x\le l_x,\, -l_y\le y\le l_y,\, 0\le z\le l_z\rbrace \) 
according to
\(H=&\int _{\text{Box volume}} \vec{A}\cdot \vec{B}\,\mathrm {d}^3\vec{r} \nonumber \\&+\int _{\text{Side faces}}\phi _{\mathrm {C}}\vec{A}_{\mathrm {C}} \, \mathrm {d}\vec{S}+\int _{\text{Top face}} ( \vec{A} \times \vec{A}_{\mathrm {C}}) \, \mathrm {d}\vec{S} \,,\) 
